Support towers for wind turbine power generation units may include a base portion that sits on the ground and acts as part of the foundation as well as a tower sitting atop the base. The support towers are subject to forces from the wind that must be resisted to prevent the support tower from being toppled. These wind forces and associated resistance of the support tower create compressive and tensile stresses within the support tower structure. These stresses must be accounted for in the design of the support tower in order to prevent fatigue failure that may result from these stresses occurring over time.
Consideration must also be given to the logistics of creating such a large structure given the limitations of the transportation infrastructure used to transport the support tower from a point of manufacture to the final site where the support tower will be used. Specifically, roadways and railways are not designed for the transportation of large support towers. One solution to this has been to create modular support towers with individual modules of various materials that can be shipped using the existing infrastructure and then assembled on site. However, this sort of transportation is still difficult and assembling the components can be complex and time consuming.
Another solution has been to cast a concrete support tower in place using methods such as slip-form casting. In this method concrete is poured into an annular form wherein it hardens. The form is moved upward and the process repeated until the tower is completed. However, this process is expensive, requires substantial labor and equipment, and is not well suited to handle changing diameters of the support tower. However, this technique has several advantages. When the base is cast in place like in this method, the base can have a much larger diameter than bases that are shipped and assembled. For this reason the base can be substantially heavier as well.
Reinforcing tendons are often used in conjunction with the cast concrete when the support tower is cast in place. The tendons may span from the foundation to a top of the support tower and may be placed in tension. Since the tendons are secured to the support tower at a bottom of the base and at the top, the tension creates a compressive load on the concrete of the support tower. This pre-stressing arrangement takes advantage of concrete's inherent superior compressive strength and reduces the times the concrete must utilize its inferior tensile strength to resist the wind forces. Conventionally, these tendons are placed within the concrete forming the support tower.
In all of these methods the base has been supported with an extremely substantial footer intended provide a dual role of preventing the support tower from sinking and providing resistance to the lateral wind forces.